Following extended operation, the images appearing on a screen of a cathode-ray tube grow paler, until they fade entirely away, possibly due to the growth of an insulating film on the tube cathode due to which the emission of electrons in an amount sufficient to cause satisfactory fluorescence of the associated screen is prevented. Such depleted cathode-ray tubes are used, disposed of and replaced by new tubes.
TV-set and cathode-ray tube makers sometimes use expensive and complex devices which allow the image of a cathode to be projected on a cathode-ray tube screen, whereby to check the functional efficiency of the tubes by visual control. This is only partially useful.